The disclosure relates, in general, to methods for joining incompatible materials, and more particularly to a system and method for enabling laminates to be welded to traditional materials such as steel.
Welding is a process in which two or more work pieces are joined by transforming at least one of the work pieces and, optionally, a filler material into a molten state to provide a weld pool. The weld pool is then cooled to form a strong bond or joint between the work pieces. In one aspect, this joining process can additionally involve heat, pressure or a combination thereof. Examples of materials that can be joined through welding include metals such as steel, aluminum, copper, and titanium, as well as thermoplastics such as acrylic, Nylon, PVC and Teflon.
Weldability refers to the ease or difficulty of welding a given set of work pieces with a certain process and a specified procedure to obtain acceptable welds. One definition of weldability according to the American Welding Society is, “the capacity of a material to be welded under the imposed fabrication conditions into a specific, suitably designed structure, and to perform satisfactorily in the intended service.” In general, if the procedure is simple, the material can be considered easily weldable. If special precautions, such as preheating, specified heat input, controlled cooling, and postheating are required, the material generally is considered not so easily weldable.
While certain combinations of materials can be joined relatively easily via welding, other material combinations can prove to be more difficult. Generally, challenges may arise if the combined metallurgy of each of the original materials prevents the production of sound joints. Unsound joints result from differences in melting temperatures, lack of appreciable solubility of either metal in the other in the solid state, and formation of brittle intermetallic compounds. In addition, stresses can develop in the weld joint due to differences in thermal expansion coefficients, thermal conductivities, and specific heats of the materials.
In one example, welding of either titanium or aluminum with other metals such as steel can be difficult due to issues such as embrittlement caused by the formation of intermetallic phases. Attempts to weld such materials without the use of specialized techniques generally results in welds of limited ductility. Table 1 provides data as reported by Sassani et al., for friction welding of combinations of materials (Sassani et al., 1988. Welding Journal 67(11): 264-s to 270-s). In the particular case of steel and titanium, the low solubility of iron in alpha titanium at room temperature results in a weld wherein the resulting intermetallic phases (i.e., TiFe and TiFe2) are very hard and brittle, thereby preventing the production of functional welds.
TABLE 1(Sassani et al.)Material CombinationType of Weld FormedAluminum alloys/magnesium alloysNo weldBrass/copperNo weldBronze/plain carbon steelNo weldBronze/steel alloyNo weldMagnesium alloys/magnesium alloysNo weldMagnesium alloys/stainless steelNo weldNickel/titaniumNo weldNiobium/stainless steelNo weldNiobium/zirconium alloysNo weldSilver/titaniumNo weldPlain carbon steel/titaniumNo weldPlain carbon steel/tungsten carbide, cementedBrittle weldStainless steel/titaniumBrittle weldStainless steel/zirconium alloyBrittle weld
Issues with forming suitable welds between work pieces or material can also arise for laminates. Laminate structures are created by stacking layers of different material in a variety of configurations followed by the application of heat and pressure to react or otherwise bond the layers together. A reliance on laminates for a particular project may be useful as these materials generally have the advantage of being both strong and light weight. Although there is a variety of material combinations used in the formation of laminates, oftentimes the resulting laminate structure cannot be directly welded to traditional steel. As a result, it may be difficult to incorporate laminates into structures of which the majority of the structural components are steel. While current methods of incorporating laminates require adhesive bonding or mechanical fastening (e.g., bolting, riveting, and the like), it may be useful to identify methods by which laminates could be welded to steel and other materials. However, more specialized techniques are required to use a welding technique to bond two or more incompatible materials.
In one aspect, techniques have been developed to isolate the incompatible materials from one another during the welding process. The two most common methods of facilitating welding and, in particular, arc welding of materials such as laminates, aluminum and, steel include the use of bimetallic transition inserts and the coating of the dissimilar material prior to welding.
Bimetallic transition materials are generally sections of material that comprise one material that has been bonded to another. Instead of arc welding, methods used for bonding the incompatible materials together can include rolling, explosion welding, friction welding, flash welding and hot pressure welding. The bimetallic transition materials can then be used as inserts to bridge two incompatible materials in normal arc welding procedures. For example, for an aluminum-steel bimetallic transition material insert, one side of the insert is welded steel-to-steel and the other aluminum-to-aluminum. One drawback is that care must be taken to avoid overheating of the insert during welding, which can result in undesirable brittle intermetallic compounds at the interface of the transition insert. Moreover, the selection of a transition insert is further complicated when one of the work pieces to be welded is a heterogeneous laminate.
In another aspect, a coating can be applied to the first material (e.g., steel) to facilitate arc welding to a second material (e.g., aluminum). One method is to coat the first material with the second material using a techniques such as dip coating (e.g., hot dip aluminizing), or brazing. Thereafter, the coated first material can be welded to the second material. However, this process also has a number of drawbacks as certain precautions must still be taken during the welding process. In particular, the second (uncoated) material should be used to form the weld pool. Alternatively, the first material can be coated with a third material that is compatible with the second material. For example, a steel surface can be coated with silver solder for welding to aluminum using aluminum filler alloy. Nevertheless, coating type joint methods are usually used for sealing purposes only and are generally not applicable when is desirable to achieve a full mechanical strength joint.
Given the aforementioned disadvantages of currently available methods for joining incompatible materials, there is a need for a system and method for enabling the welding of such incompatible materials and, in particular, the welding of laminates with traditional materials such as steel.